Especially with heavy-duty equipment such as utility vehicles it is often necessary to be able to shift among more than the usual three or four speed ratios or "gears" available in the transmission of passenger cars. For this purpose it is known to provide a power train with a plurality of cascaded ratio-changing transmission units or couplers including a basic coupler with m speed ratios and a group coupler with n speed ratios, m being generally higher than n. This enables the selective establishment of n (usually two) groups of m speed ratios each, giving a total of m.n different overall speed ratios.
During progressive upshifting from minimum speed ratio ("first gear") to maximum speed ratio (e.g. "direct drive"), the group coupler is initially in its lowest position (corresponding to maximum step-down) while the basic coupler is being successively switched from "first gear" through "m.sup.th gear". The next step calls for a return of the basic coupler to "low" while the group coupler is advanced into its second lowest position; this requires a simultaneous disengagement of both couplers for a brief period. When the system is under heavy load, as where a vehicle is being driven uphill, this interruption may cause a significant change in the relative speeds of the two shafts which could interfere with a smooth changeover to the next-higher speed ratio. An analogous situation exists during progressive downshifting.
It has therefore already been proposed, e.g. as described in U.S. Pat. No. 3,802,293, to provide a second power train bypassing the cascaded couplers in order to prevent an excessive slip in the relative shaft speeds. In the known system the bypass consists of a jaw clutch whose members are normally disengaged but can be engaged with a speed ratio corresponding to "m.sup.th gear" of the basic coupler and "low" of the group coupler, with m=6 in the example described in that U.S. patent where the group coupler has only two positions; the same speed ratio is obtained when the basic coupler is in "first gear" and the group coupler is in "high". The total number of available overall speed ratios is therefore m.n-1, or 11 in the specific example.